In a typical cellular network, also referred to as a wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation, 3G, mobile communication system, which evolved from the second generation, 2G, mobile communication system GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In the context of this disclosure, a base station as described above will be referred to as a base station or a Radio Base Station (RBS). A user equipment as described above, will in this disclosure be referred to as a user equipment or a UE.
The expression DownLink (DL) will be used for the transmission path from the base station to the user equipment. The expression UpLink (UL) will be used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage. In 3GPP, standardization body technologies like GSM, High Speed Packet Access (HSPA) and LTE have been and are currently developed.
The concept of a cellular system is that it has a large number of base stations covering a geographical area, providing mobility to the user equipments. Therefore, it is a requirement of the cellular network that, as a user equipment moves from one cell to another, it must be possible to hand an ongoing call or data session over from one base station to another with small disruptions. A handover is hence a change of serving cell for the user equipment, from a so called source cell to a so called target cell. There is a time period during the handover execution when neither the source cell nor the target cell can reach the user equipment. This will in the following be referred to as a handover outage.
Transmission Control Protocol (TCP) is a reliable, connection-oriented protocol used in the cellular network for transferring data in segments from a sender node to a destination node. Data and/or acknowledgement (ACK) of receipt of data are transferred in so called TCP packets, which comprises a header and a number of data bits.
The reliability of TCP is achieved by the sender node, such as a user equipment or a server, assigning a sequence number to each TCP packet it sends in a session to a destination node which may also be for example a user equipment or a server. The sequence number is indicated in the header of the TCP packet. When the sender node sends the TCP packet to the destination node, it starts a timer and waits for the destination node to acknowledge receipt of the TCP packet.
The time period from when the TCP packet is sent until its acknowledgment is received by the sender node is the TCP packets so called Round Trip Time, RTT.
If the acknowledgement is not received in the sender node within a pre-defined timer period, which is determined by the sender node based on RTTs for previously transferred TCP packets, a so called TCP timeout may occur in the sender node.
A TCP timeout may result in the sending node going into a so called slow start in which it retransmits the unacknowledged TCP packet, using the same sequence number, and waits for it to be acknowledged before a new TCP packet in the same session is transferred. The pre-defined timer period is called Retransmission Time Out (RTO).
A problem is that TCP sessions to or from a user equipment may be interrupted by TCP timeouts that occur in relation to handovers of the user equipment. This is because the handover outage may prevent TCP packets and/or acknowledgements from being transmitted to or from the user equipment in due time, which may hence result in the RTO being exceeded, and a TCP timeout to occur. This results in unsatisfactory performance and throughput for the TCP sessions.